The continual drive to create smaller, lighter, and thinner portable electronic devices, such as cellular telephones, requires that components, and especially the printed circuit board, must not limit the shape and size of these devices. The printed circuit board (PCB) must occupy minimal volume and accommodate the latest high density chip scale component packages. Multilayer PCBs route signals from one layer to another by means of vias, creating a compact, high density substrate. Currently, a designer does not have complete freedom to specify a layer X to layer Y connection, but is restricted to a few cost-effective via options such as mechanical and laser drilling, sequential lamination, and build up. Mechanically drilled vias can penetrate the entire PCB, but they occupy space on every layer. Laser drilled blind microvias can be employed to reduce the size and cost of the PCB, and, compared to mechanical drilling, laser drilling allows much smaller vias, and hence smaller via capture pads. Unfortunately, in a build-up construction, laser formed blind vias can only connect the outermost layer to the next inner layer. Additional size reduction could be achieved with solidly plated microvias, which allows for via stacking and fan-out for high density chip scale component packages. But solid vias require specialized plating techniques, and are only possible for vias less than 4 mils in diameter and in dielectrics less than 2 mils thick. Therefore, many board designs continue to use mechanical blind vias, foregoing the size reduction advantages of laser microvias.
Additionally, buried vias (the plated holes in the interior layers of a PCB) provide z-axis interconnection, and do not consume real estate on the outer layers of the PCB. Buried vias have often been combined with laser drilled microvias, but it is difficult to obtain equal and uniform dielectric thickness on either side of the central core when the outer layers are laminated to the core. This is because resin from the various layers does not fill the buried vias in a predictable, balanced, or uniform manner. Resin flow can vary so much that the resin thickness on the top and bottom layers can differ by as much as 2 mils, which is unacceptable for circuits that have critical impedance requirements. In addition, inconsistent resin flow can cause dimples and voids, which are also unacceptable.
It is therefore highly desirable to find a means of creating high density microvias in multi-layer PCBs for use in radio frequency (RF) applications and other circuits that have critical impedance requirements.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.